1. FIELD OF THE INVENTION
This invention pertains to a steam trap equipped with automatic drain and vent means configured to obviate the often needed external bypass, to eliminate shutdown corrosion, air vent fatigue, optionally, exit passage erosion, and to enhance the removal of sediments from the body.
Some of the embodiments of this invention pertain to a steam trap with a valve arranged at the bottom of the trap body which, during normal operation, is controlled by a float, during shutdown and start-up is opened by a thermostatic device and is also controlled by said thermostatic device when non-condensible gases accumulate above the condensate level.
2. BACKGROUND AND PRIOR ART
Present float-type steam traps are typically provided with a thermostatic air vent. Therefore, most of the discussion will address such Float and Thermostatic (F&T) steam traps, although several of the innovations described herein are applicable to other types of steam traps as well. The problems addressed by this invention are related to corrosion, failure of the thermostatic vent, the handling of corrosion product accumulations, the need for trap bypass, and exit passage erosion, all inherent in present designs. Most present designs also tend to have failure modes which are not self-annunciating, thus go undetected for long periods. The hidden leakages into the common exit lines result in large losses of steam and in difficulties with the condensate handling equipment.
Shut-down corrosion in float-type steam traps occurs because they are designed to retain condensate below a certain level in the body, and because sensitive parts of the device are not designed to prevent condensate retention during shut-down. Since most heating systems allow the entry of air during shut-down as the steam pressure drops below atmospheric pressure, the water and air together attack the sensitive parts. In addition, in F&T steam traps the thermostatic vent, connected to the exit passage and open during shut-down, allows the reentry of warm condensate vapors from the condensate return system. This, too, supports corrosive interaction with the system components. Corrosion, in addition to destroying surface and component integrity, also produces corrosion products which often interfere with the operation of system components and also can be the source of further corrosion.
Thermostatic vents often fail due to vibrations transmitted to the bellows and to fatigue.
It is well known that steam and condensate wash away corrosion products, scale and other solids, while flowing through a steam heating system. The larger pieces of these are screened out by strainers, usually arranged before steam traps. Fine rust and other minute particles, however, pass through the screens of the strainers and settle at the bottom of float-type steam traps. Accumulations of these fine particles often obstruct the free motion of the floats of steam traps, leading to their malfunction.
Some F&T traps are installed with manual bypass valves in order to accelerate the removal of the air and the excessive amount of condensate present in the piping during start-up. These valves are supposed to be opened by the personnel for the duration of the start-up; however, they are often left open inadvertently or on purpose during the operating periods. They are also subject to shutdown corrosion. Both of the above conditions contribute to the hidden system failures.
The air vents of most current F&T traps are connected to the exit passage. In some designs the exit passage is directly connected to the condensate return lines. Here vapors can re-enter the trap during shutdown and cause corrosion. In other designs an integral water seal is provided; here the corrosion of the flange between the internal passages can lead to hidden failures.
A common damage mechanism in existing steam traps is exit passage erosion. This is due to the labyrinthine exit passages, designed to minimize the trap dimensions. The high velocity caused by the condensate flashing in the low pressure region results in the entrainment of water droplets. The fast moving droplets erode the components of the exit passage. Such erosion has been observed across from the float valve and around it, the latter due to turbulence.
In the present art, an automatic drain valve is available to protect the steam traps from frost damage. Because of its limited purpose, this is available only with a small opening and is actuated either by thermostatic action or by differential pressure. Therefore it does not provide adequate bypass capacity for the air and condensate during start-up, and it is known to be prone to clogging by corrosion products and to cycling during low duty cycle operation when the condensate cools down.